How do you make color blends in glaze

Sometimes I see pieces with different color blends in glaze, it's like one color runs smoothly into another one usually in a chaotic manner. It's hard to explain, here's a picture of what I mean:

I have clear glaze and some color pigments. What's the process of doing this things? Do I have to stir a pigment with water, apply it to a piece and then cover with glaze? Or do I have to mix a pigment with glaze and apply it in one layer and then do the same for other colors?

The glaze with blue dripping color has the pigment or colorant in it. The green glazes is possibly copper or iron as the colorant and the leaf patterns may be a black stain or metal oxide in a texture applied before the glaze. I say this because there is some variation of the black where is seems to be bleeding out.
Marcia